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13/04/2026

🚀 Role of PLMN ID in Registration 🚀

1. 📡 UE PLMN Selection
✔️ UE scans SIB1
✔️ SIB1 contains a list of available PLMNs
✔️ UE selects one PLMN (operator network)
✔️ UE includes selected PLMN index → RRC Setup Complete

2. 🏗️ gNB Processing (Routing Decision Point)

🔹 A. AMF Selection
✔️ gNB reads PLMN ID (+ optional S-NSSAI)
✔️ Uses it to select correct AMF pool

Behavior:
✅ If UE is new (no 5G-GUTI):
→ gNB selects AMF based on PLMN + slice
❌ If no matching AMF exists:
→ triggers RRC Release or NGAP Setup Failure

🔹 B. RAN Sharing (MOCN Scenario)
✔️ One gNB connected to multiple cores:
Core A → MCC 123 / MNC 01
Core B → MCC 123 / MNC 02
✔️ The gNB reads PLMN Index and ensures the Initial UE Message is sent over the correct SCTP association to the corresponding operator's AMF.

3. 🛡️ AMF Validation : Consistency Check

🔹 AMF verifies: PLMN ID from RRC/NGAP vs Serving PLMN in NAS Registration Request

🚨 Mismatch Handling : If mismatch occurs, AMF rejects UE with cause: 5GMM #11 – PLMN not allowed

13/04/2026

To visualize protocols interaction for UE Registration Request.

NAS Layer (TS 24.501): Generates the Registration Request.

RRC Layer (TS 38.331): Wraps the NAS PDU into an RRCSetupComplete and sends it over the SRB1 (Signaling Radio Bearer 1).

NGAP Layer (TS 38.413): The gNB receives it, strips the RRC headers, and wraps the NAS PDU into an Initial UE Message for the AMF.

11/04/2026

🚀 Why is the User Location Information (ULI) in the Initial UE Message so critical?

It’s not just for paging! 👀👀

The gNB makes the first choice of AMF, but the ULI (TAI/Cell ID) allows the NSSF to perform a final audit. If the location data shows the current AMF doesn't support the requested slice in that specific zone, the ULI triggers AMF Reselection (AMF Re-allocation). No ULI = No accurate slicing validation!

Easy but important for right view of IE (Information Element).

09/04/2026
09/04/2026

5G NAS Messages - Readable Vs Encrypted

09/04/2026

5G SA Registration & Failure

09/04/2026

5G SA Registration cause and some failure

09/04/2026

5G SA Registration - short video

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